If its the screw on lens your on about,it's hard to find on its own in a shop,best bet is ebay people put old Broken lamps on there,it'll do the same job.
I personally don't see a big difference in 140 to 170.
My a pinyon if you have a day dog that's,taking his time or tying to run to clever,I think a few rabbits on the lamp do sharpen um up,and I have had day dog with too much lamping for some reason stops at hedges,when before hand wood go strait threw